A typical recording media records data in the form of transitions between two opposite states. In magnetic recording media, such as magnetic tape, the transitions are between opposite magnetic polarities. Data to be provided to a recording channel is typically a sequential string of bits, to be recorded on the recording media as transitions. A typical example of encoded data is run length limited (RLL) encoded data. In RLL encoding, transitions typically represent "1" bits and are separated by spaces which represent "0" bits. The distance between transitions therefore represents the number of "0" bits, and are detected by counting the number of clocks between the transitions. The clocks are not recorded with the data and must be determined from the data itself.
Thus, a data set will include a VFO ("VFO" stands for variable frequency oscillator) pattern at a specific location (typically a header) and of a particular known transition frequency to synchronize the read clock (typically a phase locked loop or PLL) to the codeword bit frequency. The VFO pattern is typically a repetitive codeword pattern, such as a sequence of "10" or 2T bits, but which is identifiable because of its location in a header. A known sync pattern is typically also provided between the VFO pattern and any encoded data to allow the decoder to align to the codeword boundaries and to the start location of the incoming data.
If the PLL does not achieve complete phase lock, or if the decoder does not align to the codeword boundaries on the incoming data (either of which may happen if a media defect is present), the decoder will not be able to successfully decode incoming encoded data, until the next Sync or Resync pattern is encountered, usually in the header of the next data set or in a header for a grouping of codeword groups. When a Sync pattern is not detected (missed) or detected erroneously in the wrong position (e.g., due to a defect), the exact bit position and alignment to codeword boundaries is not known. Accordingly, the decoder will not be able to successfully decode the encoded data unless or until some further synchronization field is encountered, such as the next Sync or Resync. This is known as infinite error propagation, and this type of failure can be catastrophic.
Even though the location of the VFO pattern and the synchronization pattern are known, a difficulty is that misdetection of one or more bits in the synchronization pattern may prevent recognition of the synchronization pattern. Without proper recognition of the synchronization pattern, the data detector will not be in sync with the recorded data and may be unable to recognize the data.
Errors in the data are typically handled by using well-known error correcting codes. Such codes allow detection and correction of many data recording errors. However, errors in the VFO or synchronizing patterns may result in missing detection of the synchronization pattern or detecting it in the wrong place. Therefore, the data detection may begin with reading data before or after the data really starts. If the start of data is not determined via detection of the synchronization mark, all data between it and any subsequent synchronization mark which is detected successfully will be lost. This may have a catastrophic effect, rendering useless the error correcting codes protecting the data.